A basic advantage for an all plastic PAR lamp is its light weight as compared to the glass construction now in use. Consequently, a minimum wall thickness for said plastic PAR lamp construction is desirable to provide as little weight as possible while still recognizing that heat and mechanical distortion of the plastic material both during lamp assembly and thereafter in use can give rise to a number of serious problems. Such a problem is experienced during assembly of an all-plastic PAR lamp for an automotive headlamp construction wherein accurate positioning of the light source at the focus of the reflector member is understandably desirable. Any heat or mechanical distortion of the parabolic shaped cavity in such a reflector member can preclude accurate focusing when either ultrasonic or vibrational welding techniques are employed in joining the mount light source to the reflector member as a means of providing a leak-proof seal therebetween. Any subsequent heat distortion of the assembled construction during lamp operation at elevated temperatures can cause much the same problem with respect to continued accurate focusing of a light source in said reflector member. Consequently, a more reliable positioning means of a light source in an all plastic reflector member is desirable which permits accurate focusing of the light source both during lamp assembly as well as subsequent lamp operation.